Crystalline substances have a definite rigid shape. The shape and size of crystals (even of the same materials) differs depending upon the conditions under which they are grown. Crystals of a given substance are bound by plane surfaces called faces. The angle between any two faces is called interfacial angle. But the angles between the faces of a given form always remains same. This important characteristic feature of a given crystalline substance is known as law of constancy of interfacial angles.
Types of Crystalline Solids
Crystalline solids may be classified into four types depending upon the nature of bonds present in them.
Molecular crystals
In molecular crystals, the constituent particles are molecules. These molecules are held together by weak forces known as Van der Waal's forces. Common examples are dry ice, wax, iodine, sulphur, etc.
Characteristic features of molecular crystals are:
Molecular crystals are soft, compressible and can be distorted very easily.
They have low melting and boiling points.
These are bad conductors of electricity and are regarded as electrical insulators.
They are volatile and have low heats of vaporization and low enthalpy of fusion.
Ionic crystals
The ionic crystals consist of positively and negatively charged ions arranged in a regular fashion throughout the crystal. They form a network of positive and nagative ions in three dimension in such a way that cations and anions occupy alternate sites. These are held together by strong electrostatic forces.
The main characteristics of ionic crystals are:
Ionic crystals are very hard and brittle.
They have very high melting and boiling points.
They are poor conductors of electricity and therefore are insulators in the solid state.
They have high heat of vaporization and so have low vapour pressure.
Have high enthalpy of fusion.
When melted or dissolved in polar solvents, they conduct elecricity.
Common examples of ionic crystals are, salts like NaCl, KNO3, LiF etc.
Covalent crystals
In covalent crystals, the constituent particles are atoms of the same or different kind, which are bonded to one another by a network of covalent bonds. The important characteristics of covalent crystals are:
The covalent crystals are incompressible and hard.
They are extremely non-volatile and have very high melting points.
They are poor conductors of electricity at all temperatures.
They have high heat of fusion and high enthalpy of atomization.
The common examples of covalent crystals are diamond, carborundum (silicon carbide), quartz (SiO2,) etc.
Metallic crystals
In metallic crystals the constituent particles are positive kernels i.e., nuclei where inner electrons are dispersed in a sea of mobile valence electrons. The forces present between the constituents are metallic bonds.
The main characteristics of metallic crystals are:
Metallic crystals may be hard as well as soft.
They are good conductors of heat and electricity.
They have metallic luster and high reflectivity.
They are highly ductile and malleable i.e, they can be beaten into sheets and drawn into wires.
They have moderate heat of fusion.
The properties of metallic crystals vary from metal to metal.
The examples of metallic crystals are common metals such as nickel, copper and alloys.
Problem
12. The melting point is a rough measure of the attractive forces in solids.
Arrange the following solids in the order of increasing strength of attractive forces.
| | m.p.(k) |
| Naphthalene | 353 |
| Sodium fluoride | 1272 |
| Water (ice) | 273 |
| phosphorus | 317 |
| Zinc iodide | 719 |
Solution
The order of increasing strength of attractive forces is: Water ³ phosphorus ³ naphthalene ³ zinc iodide ³ sodium fluoride.
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